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Comparison of ^99mTc-Methoxyisobutyl Isonitrile and ²⁰¹Tl Scintigraphy in Visualization of Suppressed Thyroid Tissue

Tanju Yusuf Erdil, Çetin Önsel, Bedii Kanmaz, Biray Caner, Kerim Sönmezolu, smail Çiftçi, Turgut Turolu, Levent Kabasakal, Haluk Burçak Sayman and lhami Uslu

Department of Nuclear Medicine, Marmara University School of Medicine, Istanbul
Department of Nuclear Medicine, Cerrahpaa Medical Faculty, Istanbul University, Istanbul
Department of Nuclear Medicine, Hacettepe University School of Medicine, Ankara, Turkey

Correspondence: For correspondence contact: Tanju Yusuf Erdil, MD, Dikmen Cad. 123/9, Dikmen, Ankara, 06450 Turkey.

ABSTRACT

Both ²⁰¹Tl and ^99mTc-methoxyisobutyl isonitrile (MIBI) have been used in the visualization of suppressed thyroid tissue in patients with autonomously functioning thyroid nodules (AFTNs). It has been suggested that thyroid-stimulating hormone (TSH) control is not a major determinant of both tracers. However, the mechanism of thyroid uptake of these agents is controversial. In this study, we compared ²⁰¹Tl and MIBI in the visualization of suppressed thyroid tissue in patients with a solitary toxic AFTN. Methods: Thirty-two patients (13 triiodothyronine [T₃] and 19 T₃ + levorotatory thyroxine [T₄] hyperthyroid patients) with toxic AFTNs visualized on ^99mTc-pertechnetate scanning were included in the study. All patients underwent MIBI and ²⁰¹Tl thyroid scintigraphy within a 3-d interval. The scintigrams were analyzed both visually and semiquantitatively. For the semiquantitative analysis, regions of interest (ROIs) were generated over the nodule (N) and contralateral normal lobe (E), and the mean counts in each ROI were calculated. Results: The N/E uptakes (mean ± SD) for pertechnetate, MIBI, and ²⁰¹Tl were 11.37 ± 4.53, 4.76 ± 1.38, and 1.63 ± 0.15, respectively, in T₃ + T₄ hyperthyroid patients and 9.46 ± 3.64, 2.73 ± 0.63, and 1.57 ± 0.23, respectively, in T₃ hyperthyroid patients. Our results showed that ²⁰¹Tl uptake of suppressed thyroid tissue compared with AFTN was more prominent and significantly higher than that of MIBI for both groups of patients (P = 1.08E-05 for T₃ and 6.15E-09 for T₃ + T₄ hyperthyroidism). There was no significant difference for either pertechnetate or ²⁰¹Tl (P > 0.05) when the N/E uptakes of both groups of patients were compared. However, the N/E uptake of MIBI in T₃ + T₄ hyperthyroid patients was significantly higher than that in T₃ hyperthyroid patients (P = 6.69E-06). Conclusion: Clear visualization of suppressed thyroid tissue with both ²⁰¹Tl and MIBI in patients with low serum concentrations of TSH suggests that TSH is not a major factor in the thyroid uptake of either agent. ²⁰¹Tl is superior to MIBI in the visualization of suppressed thyroid tissue in patients with a toxic thyroid nodule. An increased rate of metabolism in the follicular cells of AFTNs in T₃ + T₄ hyperthyroid patients compared with that in T₃ hyperthyroid patients might be responsible for the higher N/E for MIBI compared with that for ²⁰¹Tl.

Key Words: thyroid nodule • ^99mTc-MIBI • ²⁰¹Tl • TSH

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